The present invention relates to nonvolatile memories. More particularly, the present invention relates to floating gate nonvolatile memory circuits and methods.
Generally, memory circuits are used to store information in an electronic system. Typically, information is stored as binary data (e.g., 0's and 1's) represented in the system as binary values of voltages or currents. While many semiconductor memory architectures exist, they generally can be categorized as volatile and nonvolatile. Volatile memories are those memories that require a periodic refresh of the data values stored electronically in the memory. One example of a volatile memory is a dynamic random access memory, wherein data may be stored as a voltage on a capacitor. However, because the voltage on the capacitor may dissipate over time, such memories require a periodic refresh, wherein the voltage on the capacitor is refreshed to its nominal value. Additionally, all the information stored in such memories is typically lost when a power source is removed from the system. Nonvolatile memories, on other the other hand, include all forms of solid state memory that do not need to have their memory contents periodically refreshed. This includes all forms of programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), and flash memory.
Nonvolatile memory circuits are advantageous over volatile memory circuits because they have the ability to store data without the need for a constant source of power. Many nonvolatile memories take advantage of various electrical phenomena to move electrons to and from an isolated conductor. The isolated conductor is often referred to as a floating gate. When the electrons are moved to the isolated conductor, the voltage on the conductor decreases, and when the electrons are moved from the isolated conductor, the voltage on the conductor increases. The change in voltage may be used as a binary representation of data. Therefore, the voltage changes may be detected and the data values they represent may be used to control other electronic circuits in the system.
However, one problem with existing nonvolatile memories is the relatively large voltages that must be generated in order to move electrons to and from the isolated conductor. Electronic circuits typically have a nominal power supply voltage, and if the voltage required for operating a nonvolatile memory element exceeds the nominal supply voltage, a variety of problems can occur. One immediately evident problem is the large voltages may exceed the breakdown voltages of other devices in the system. Another problem pertains to the complexity of the circuitry required for generating the high voltages.
Yet another problem with existing nonvolatile memories is the cost and complexity of the processes that must be used to implement such memories. Existing nonvolatile memories may require very complicated semiconductor processing techniques with many process steps. However, as the semiconductor process becomes more complicated, the cost of the process tends to increase. Additionally, complicated processes also tend to result in lower yields (i.e., higher defect rates), thereby reducing the profitability of any circuits manufactured on the process.
Therefore, what is needed are more effective circuits and methods for implementing nonvolatile memories.